Program

/* spontaneous decay simulation */

#include <stdio.h>
#include <math.h>

#define lambda 0.01			/* the decay constant */
#define max 1000			/* number of atoms at t=0 */
#define time_max 500			/* time range */
#define seed 11168			/* seed for number generator */

FILE *output;				/* internal file name */

main()
{

int atom, time, number, nloop;
double decay;

number=nloop=max;				/* initial value */

output = fopen("decay.dat", "w");	/* external file name */

srand48(seed);				/* seed the number generator */

for (time=0; time<=time_max; time++)		/* time loop */
{
   for (atom=1; atom<=number; atom++)		/* atom loop */
   {
      decay=drand48();
         
      if (decay<lambda) nloop--;		/* an atom decays */
    }
    number=nloop;
    
    fprintf(output, "%d\t%f\n", time, (double) number/max);	
}

fclose (output); 
}